(1) Technical Field
This invention relates to electronic radio frequency (RF) circuits, and more particularly to programmable multi-reflective RF phase shifter circuits.
(2) Background
It is often necessary to change the phase of RF signals for applications such as in-phase discriminators, beam forming networks, power dividers, linearization of power amplifiers, and phase array antennas, to name a few. RF phase shifters are electronic circuits which can change the phase of an RF signal. One type of RF phase shifter comprises a hybrid coupler and a pair of reflective terminating circuits.
A hybrid coupler is a passive device often used in radio and telecommunications. It is a type of directional coupler where the input power is equally divided between two output ports by electromagnetic coupling; accordingly, it is often called a 3 dB coupler. FIG. 1 is a schematic diagram of a hybrid coupler 100. As is known in the art, directional couplers have four ports. The Input Port is where power is applied. The Coupled Port is where a portion of the power applied to the Input Port appears. The Direct Port is where the power from the Input Port is output, less the portion of the power that went to the Coupled Port. Directional couplers are generally symmetrical, so there also exists a fourth port, the Isolated Port, which is isolated from the Input Port.
One example of a hybrid coupler 100 is the Lange coupler, a four-port, interdigitated structure developed by Dr. Julius Lange around 1969. The electromagnetic coupling among ports in a Lange coupler is derived from closely spaced transmission lines, such as microstrip lines. Lange couplers come in a variety of embodiments and are widely used as power combiners and splitters in RF amplifiers as well as in mixers and modulators. Further details regarding Lange couplers may be found in U.S. Pat. No. 3,516,024, issued on Jun. 2, 1970 to Lange for an “Interdigitated Strip Line Coupler”, and also described in Lange, “Interdigitated Strip-Line Quadrature Hybrid”, MTTS Digest of Technical Papers, Dallas, Tex., May 5-7, 1969, pp. 10-13.
A hybrid coupler can be categorized by five main parameters: bandwidth, insertion loss, coupling ratio, phase shift between ports, and isolation. Bandwidth is defined as the frequency range where the device provides a phase shift within ±10 degrees of the desired phase shift. Insertion loss is the additional loss within the coupler above the loss due to signal splitting. This can be caused by reflections of signals, dielectric losses, and conductor losses. Coupling ratio is defined as the ratio of the lower of the two output powers to the input power. Isolation refers to the ratio between the input power and the leakage power at the Isolated Port.
A reflection-based phase shifter relies on generally identical reactive elements connected to the Direct Port and the Coupled Port of a hybrid coupler. FIG. 2 is a schematic diagram of a reflection-type phase shifter 200 that comprises a broad-band hybrid coupler 100 and a pair of reactive elements in the form of reflective terminating circuits 202, 203. An ideal reactive element has a reflection coefficient of unity (i.e., an ideal non-lossy element). The reflective terminating circuits 200 connected to the Direct Port and the Coupled Port provide reflections of an input signal presented at the Input Port (RF In) which cancel on the Input Port and sum to a phase-shifted version of the input signal on the Isolated Port (RF Out).
By varying the reactance of the reflective terminating circuits 202, 203, the degree of phase shift provided by the reflection-type phase shifter 200 can be controlled. Perhaps the simplest form of reflective terminating circuit 202, 203 is a capacitor. If an arrangement is made to switch the capacitor ON or OFF, then a relative phase shift of Δ∅ can be achieved; expressed in terms of scattering parameters (S-parameters) and phase angle, Δ∅ is:Δ∅=S11∠∅(ON)−S11∠∅(OFF)where S11 is the voltage reflection coefficient of the Input Port.
FIG. 3A is a schematic diagram of a simple reflective terminating circuit 300 comprising a capacitor C0 that may be switched into or out of circuit by a field-effect transistor (FET) switch SW0 coupled to circuit ground. The FET switch SW0 is controlled by a CTRL signal supplied by other circuitry (not shown). FIG. 3B is a simplified schematic diagram of the circuit 300 of FIG. 3A in which the FET switch SW0 is represented as a single-pole, single-throw switch (the CTRL signal is implicit).
FIG. 4 is a schematic diagram of a multi-reflective terminating circuit 400 comprising three parallel capacitor/switch units 402a-402c that each comprise a capacitor C0-C2 and corresponding switch SW0-SW2; the values of the capacitors C0-C2 may vary. By selectively switching the capacitors C0-C2 in or out of circuit, the amount of reactance coupled to the hybrid coupler 100 (see FIG. 2) can be varied, thus providing a selectable amount of phase shift.
A challenge in designing multi-reflective phase shifters is achieving digitally controlled multiple equidistant phase shifts such that needed capacitance values increase uniformly, while taking into account the capacitance of each capacitor/switch unit's phase shift contribution (which is affected by loading of its own OFF state phase value as well as by the ON or OFF state of other capacitor/switch units 402a-402c). More generally, such a design challenged extends to achieving digitally controlled multiple equidistant phase shifts such that applied reactance values increase uniformly.
The present invention addresses these issues by teaching a number of novel concepts for improved multi-reflective RF phase shifters.